1) Field of the Invention
The present invention relates to an axial piston pump for pumping an Operating oil through a hydraulic system, and more particularly, it relates to such an axial piston pump having a fixed slant cam plate for causing a reciprocation of the pistons therein.
2) Description of the Related Art
In an axial piston pump of this type, the pistons are slidably received in cylinder bores formed in and around a rotational axis of a rotary cylinder block, at regular intervals, respectively. Each of the pistons has a spherical end portion projecting from the cylinder bore, and this spherical end is slidably received and held in a spherical recess formed in a shoe element. The shoe element is slidably engaged with the fixed slant cam plate, and is supported by a rotatable retaining member that maintains the slidable engagement between the fixed slant cam plate and the shoe element. This arrangement enables the pistons to be reciprocated in the bores of the cylinder block, respectively, during a rotation of the cylinder block.
Each of the cylinder bores of the rotary cylinder block is opened at the bottom thereof through a hole formed in an end wall of the cylinder block, and these holes are disposed at regular intervals along a circle having a center thereof located on the rotational axis of the cylinder block. The cylinder block is disposed in such a manner that the end wall thereof faces a fixed annular valve plate having an arcuate suction port and an arcuate discharge port, and is resiliently pressed against the valve plate so that the suction and discharge ports thereof are laid along the same circle as that around which the holes of the cylinder block are disposed. Further, the arcuate suction and discharge ports of the valve plate are positioned in such a manner that, when each of the holes of the cylinder block passes through the arcuate suction port during the rotation of the cylinder block, the piston associated with that hole is in a suction stroke, and when each of the holes of the cylinder block passes through the arcuate discharge port during the rotation of the cylinder block, the piston associated with that hole is in a discharge stroke. Therefore, when the pump is incorporated into a hydraulic system, it operates in such a way that the operating oil is sucked up through the arcuate suction port of the valve plate into the cylinder bores and is discharged from the cylinder bores through the arcuate discharge port thereof.
During the pumping operation of the pump as mentioned above, a leakage of the operating oil occurs at the contact wall surfaces between the valve plate and the cylinder block end wall, to form a lubricating oil film therebetween, but this leakage must be kept as small as possible, to ensure an efficient operation of the pump. To this end, conventionally, the valve plate has an annular seal land formed thereon and surrounding the arcuate suction and discharge ports, and the cylinder block end wall is brought into a sealing engagement with the annular seal land of the valve plate. In this arrangement, a peripheral annular groove is formed between the cylinder block end wall and the valve plate, and serves as a guide passage for the leaked operating oil. In this conventional pump, further, the cylinder block is susceptible to a vibrational play during the rotation thereof, because no support of the peripheral annular portion of the cylinder block end wall can be provided due to the presence of the peripheral annular groove of the valve plate, and accordingly when the vibrational play occurs at the rotating cylinder block, the leakage of the operating oil becomes larger and the efficiency of the pump is thus lowered.
To prevent the vibrational play of the cylinder block, it has been suggested in Unexamined Japanese Patent publication No. 47(1972)-44201 (Kokai) that a plurality of segment-shaped bearing lands be located at regularly spaced intervals along the periphery of the valve plate, to support the peripheral annular portion of the cylinder block end wall. Namely, the vibrational play of the cylinder block can be effectively prevented because the peripheral annular portion of the cylinder block end wall can be supported by the segment-shaped bearing lands, during the rotation of the cylinder block. Note, with this arrangement an annular oil passage is formed between the annular seal land and the segment-shaped bearing lands, and a plurality of radial oil passages are formed between the adjacent segment-shaped bearing lands.
Nevertheless, in this pump, the annular seal land of the valve plate suffers greater wear than the segment-shaped bearing lands thereof. In particular, since the operating oil inevitably includes very small pieces of grit such as metal powders, and these very small pieces of grit are contained in the oil films formed between the wall end of the cylinder block and the annular seal land and segment-shaped bearing lands, these small pieces of grit in the oil films act as an abrasive on the contact surfaces between the cylinder block end wall and the seal and bearing lands. The oil film formed between the cylinder block end wall and the annular seal land has a higher pressure than that of the oil films formed between the cylinder block end wall and the segment-shaped bearing lands, because these segment-shaped bearing lands are separated from the annular seal land by the annular oil passage intervened therebetween, and because they are arranged at regularly spaced intervals along the periphery of the valve plate to form the plurality of radial oil passages between the adjacent segment-shaped bearing lands. Accordingly, the contact surfaces between the cylinder block end wall and the annular seal land of the valve plate suffer more wear than the contact surfaces between the cylinder block end wall and the segment-shaped bearing lands thereof, and thus an undesirable clearance can be formed between the cylinder block end wall and the annular seal land of the valve plate during a long operation of the pump, which will inevitably lower the operating efficiency of the pump.